Method for reducing interdiffusion rates between thin film components

ABSTRACT

A method for reducing interdiffusion rates between thin film conductive components. The method comprises vacuum depositing the conductive films at a partial pressure of 10-70 nicrons in the presence of an inert gas (e.g., Argon).

United States Patent 1 1 Theuerer et a]. 1 Aug. 7, 1973 METHOD FOR REDUCING 3,063,858 ll/l962 Sleeves 117 107 INTERDIFFUSION RATES BETWEEN THIN 3,402,073 9/1968 Pierce et al 1 17/217 FILM COMPONENTS OTHER PUBLICATIONS [75] Inventors: Henry C. Theuerer, New York, I

NY; Paul Turner, Murray Hill Holland; L., Vacuum Deposition of Thm Films, N.Y., NIL John Wiley & Sons lnc., l956, pg. 4. 73 Ass'ne: BellTlehneLbo t l 1 lg e f g z z g gs J Primary ExaminerRalph S. Kendall Assistant Examiner-M. F. Esposito [22] Fil d? Apr- 4, 1969 Att0rneyW. L. Keefauver and Edwin B. Cave [21] Appl. No.: 813,421

[57] ABSTRACT 5 I E 3 S 4 22 A method for reducing interd1ffus1on rates between [58] Fie'ld 6 217 227 1 thin film conductive components. The method comprises vacuum depositing the conductive films at a par- [56] Refrences Cited tial pressure of 10-70 nicrons in the presence of an t UNITED STATES PATENTS met (e g Argon) 3,028,663 4/1962 lwersen et al l-l7/l07 5 Claims, 2 Drawing Figures PAIENIEUMIB Hm 1660 HRS H c. THEUERER MEMO P.A. TURNER B) A TTORNEV METHOD FOR REDUCING INTERDIFFUSION RATES BETWEEN THIN FILM COMPONENTS This invention relates to a technique for reducing the interdiffusion rates of thin film components. More particularly, the present invention relates to a technique for reducing the interdiffusion rates between thin film conductive components wherein the components of interest are deposited by vacuum evaporation techniques in the presence of a non-reactive gas.

In recent years, miniaturization of components and circuitry coupled with the increasing complexity of modern electronic systems have created an unprecedented demand for reliability in thin film circuitry and the need for the total exploitation of the technology. This is particularly true in the case of lead attachment which has long been recognized as being a critical factor in the stability of circuit characteristics.

Early workers in the art recognized that the metallurgical compatibility of the various metallic constituents of the joining and conducting system played a prominent role in determining the parameters of interest, so motivating the use of a single metal'for'this purpose. Although such systems were found to be ideal from a metallurgical standpoint, they suffered from inherent defects in that the manufacturer was necessarily restricted from the standpoint of obtaining optimum circuit characteristics. Accordingly, the interest of workers in the art was focused upon multi-metal joining or conducting systems. v

Unfortunately, it has been found that diffusion or the migration of metals in thin film composites through grain boundaries and down dislocation cores drastically alters device characteristics during life performance. Accordingly, workers in the art have long sought to develop techniques for enhancing the stability of thin film structures including thin film conducting composites.

In accordance withthe present invention, the prior art difficulties encountered in connection with such thin film composites have been significantly lessened by a novel technique which causes a reduction in interdiffusion rates between the components of the composite. The inventive technique involves depositing thin film conducting composites by vacuum evaporation techniques in the presence of a suitable non-reactive gas. Studies have revealed that processing in the tile. scribed manner results in the reduction of the diffusivity of metals by segregating diffusion paths and impeding the movement of diffusants by incorporation of the gas of interest in grain boundaries and dislocation cores.

The invention will be more readily understood by reference to the following detailed description taken -in conjunction with the accompanying drawing, wherein:

FIG. 1 is a front elevational view of an apparatus suitable for use in producing a film of metal by vacuum evaporation techniques in accordance with the present invention; and

FIG. 2 is a graphical representation on coordinates of time in hours against reflectivity showing diffusion rates for gold-silver thin film couples prepared in accordance with the present invention.

With reference now more particularly to the drawing, FIG. 1 shows a suitable vacuum evaporation apparatus for depositing thin films in accordance with the invention. Shown in the figure is a vacuum chamber 11 containing filament l2 and platform 13, which latter is employed as a positioning support for substrate 14. Mask 15 is utilized as shown to restrict the deposition of the film to the desired area. The ends of filament 12 are connected to electrical leads 16 to permit flow of current therethrough from a source (not shmgn). The apparatus also includes an inlet 17 for the introduction of gas during the processing and an outlet 18 for removal of gas, outlet 18 being connected to a suitable pump (not shown).

The present invention has been described largely in terms of the deposition of gold-silver composites. However, it will be appreciated that such description is solely for purposes of exposition and that any metal capable of being deposited'in thin film form by vacuum evaporation techniques may be obtained in the described manner. Metals found to be particularly useful for thin film applications and amenable to such processing include aluminum, copper, nickel, iron, Pennalloy, etc.

The invention may conveniently be described by reference to an illustrative embodiment wherein silver and gold are employed as the thin film couple. An apparatus similar to that shown in FIG. 1 may conveniently be employed for the purpose of effecting deposition of the films;

The conditions employed in the vacuum evaporation process are well known .(see Vacuum Deposition of I Thin Films, L. Holland, John Wiley & Sons, Inc., New York, I956). The extent of the vacuum is dictatedby consideration of the vapor pressurev of the metal to be evaporated. In conventional vacuum evaporation processes, it is generally considered that the vapor pressure of the metal to be evaporated should beat least ten times greater than the pressure to which the system is evacuated. In general, better quality films are-obtained at a'higher vacuum. When using metals'with relatively high vapor pressures, themaximum pressure which can be tolerated is that above which the oxygen present inte'rferes-with the deposition of a pure metallic film.

The usual method of heating the metal to be evaporated is to position it in proximity to a filament which may be heated electrically. This end is conveniently accomplished by using a tungsten filament in the shape of a coil, as shown in FIG. 1, and placing the metal to be evaporated within the coil. The required temperature is obtained by controlling the magnitude of the current flowing through the filament. Alternatively, a filament of the metal to be evaporated may be used in those instances where the metal has a sufficiently high vapor pressure at temperatures below its melting point.

It will be understood by those skilled in the art that the substrate member upon which deposition is to be effected, as described, may be selected from among glasses, glazed ceramics, high melting metals and any thin film circuitry requiring the deposition of a metal composite thereon.

The substrate member is first cleaned for the purpose of removing contaminants therefrom, any well-known cleansing technique being suitable for this purpose. Substrate 14 is then placed upon platform 13 and mask 15 suitably positioned. Next, vacuum chamber 11 is evacuated to a pressure of the order of 10"torr. .and a non-reactive gas admitted thereto.

For the purposes of the present invention,it will be understood that the term non-reactive applies to gases which do not normally enter into chemical reaction with metals. It has been found that argon, helium,

neon, xenon, krypton, radon and hydrogen meet these requirements. Studies have indicated that in order to obtain the desired reduction in diffusivity, it is necessary to employ from to 70 microns of Hg of the nonreactive gas. Deviations from the minima and maxima noted fail to yield the required characteristics and result in diffusion rates comparable to those of the prior art.

After the requisite pressure is obtained, a current is passed through tungsten filament 12, thereby heating the filament and causing the metal of interest to evaporate. Evaporation is continued for a time period sufficient to produce a thin film of the desired thickness. Thereafter, at least one additional metal is deposited in the foregoing manner upon the film so produced. The thickness of the deposited layers is of no criticality and it is solely dependent upon the specific role that the metal composite is to play in the resultant circuitry.

Examples of the present invention are described in detail below. These examples and the foregoing illustration are included merely to aid in the understanding of the invention and variations may be made by one skilled in the art without departing from the spirit and scope of the invention.

EXAMPLE NO. 1

This example describes the production of a thin film couple in accordance with the invention wherein gold and silver are successively evaporated upon a substrate.

An apparatus similar to that shown in FIG. 1 was employed, the filament being composed of a 5 turn tungsten coil. The substrate selected was a glass microscope slide which was cleaned by ultrasonic cleansing in AL- CONOX, and successively boiling in a three part hydrocloric acid-one part SUPEROXOL solution, rinsing in deionized water, vapor drying with isopropyl alcohol and baking at 560 C. for l5 minutes in air to remove residual carbonations material.

After cleaning, the slide was placed approximately 3% inches from the tungsten filament and a mask placed on the slide. A 1 inch piece of gold, 25 mils in diameter was placed within the tungsten filament. The vacuum chamber was evaporated to a pressure of approximately 1 X10 torr. and 35 microns of Hg of argon admitted to the chamber. Current was caused to flow through the tungsten filament, heating it to incandescence and causing the evaporation of the gold wire. The 1 inch piece of gold wire was completely evaporated within five minutes and a layer of gold 1500 angstroms thick was produced on the exposed portions of the slide. Then, a new tungsten coil was inserted in the apparatus and a silver wire 1 inch in length was inserted in the coil. The evaporation procedure was repeated, so resulting in the evaporation of a layer of silver 5,000

angstroms thick upon the gold layer. Optical reflectivity (which is directly related to composition) as a function of time was then measured at the gold-glass interface by conventional techniques. After 49 hours, the reflectivity was found to drop approximately 7 percent, so indicating that a small amount of silver had migrated into the gold film. Thereafter it was found that optical reflectivity remained constant for greater than 5625 hours. 7

For comparative purposes, the foregoing procedure was repeated in the absence of the argon. Optical re flectivity was found to decrease approximately 15 percent after 25 hours, 35% after 100 hours and about 45% after 400 hours. Thereafter the optical reflectivity was found to remain constant at that level for greater than 4900 hours.

EXAMPLE NO. 2

The procedure of Example 1 was repeated with the exception that helium was employed as the nonreactive gas. Optical reflectivity was found to drop approximately 7 percent after 49 hours and remained constant thereafter for more than 5625 hours.

With reference now to FIG. 2, there is shown a graphical representation on coordinates of time in hours against reflectivity in per cent showing variations in optical reflectivity as a function of time (or diffusion rates) for thin film gold-silver couples. It is noted that utilizing 35 and microns of Hg of argon or helium, respectively, as the non-reactive gas in the practice of the present invention results in only a slight decrease in reflectivity as compared with optical reflectivity changes manifested by gold-silver couples evaporated in the absence of such gases.

What is claimed is: v

1. A method for reducing interdiffusion rates between thin film conductive components which com-. prises the steps of successively depositing at least two metals capable of being deposited in thin film form by vacuum evaporation upon a suitable substrate by vacuum evaporation techniques in the presence of a nonreactive gas, selected from the group consisting of argon, helium, neon, xenon, krypton, radon and hydrogen, the partial pressure of said gas ranging from 10 to 70 microns.

2. A method in accordance with claim 1 wherein one of said conductive components is gold.

3. A method in accordance with claim 2 wherein one of said conductive components is silver.

4. A method in accordance with claim 3 wherein said gas is argon.

5. A method in accordance with claim 3 wherein said a gas is helium.

i i 0 i i 

2. A method in accordance with claim 1 wherein one of said conductive components is gold.
 3. A method in accordance with claim 2 wherein one of said conductive components is silver.
 4. A method in accordance with claim 3 wherein said gas is argon.
 5. A method in accordance with claim 3 wherein said gas is helium. 